Video-on-demand systems are known to deliver video and audio content to specific customers over a shared broadband network. A typical embodiment of a video-on-demand (VOD) system is illustrated in FIG. 1.
FIG. 1 illustrates a VOD system 10 that includes a plurality of redundant array of independent disks (RAID) 18-24, a video-on-demand distribution system 12, a plurality of quadrature amplitude modulation (QAM) modules 26-34, and a control computer 16 operably coupled to a plurality of set-top boxes 36-44 via a cable network 14. The video-on-demand distribution system 12 includes a plurality of servers 46-52 and a plurality of statistical multiplexors 54-60.
Each of the redundant array of independent disks are known to comprise a large disk storage system that provides vast amounts of data storage for storing video and audio programs in digital form. Each redundant array of independent disks also includes an interface that allows access to the data. Such an interface is typically a small computer system interface (SCSI) or an integrated drive electronics (IDE) interface, which allows data to be retrieved at rates of 14 to 30 megabytes per second. The control computer 16 is operably coupled to receive requests for access to one or more programs stored in the plurality of RAIDs 18-24 from the plurality of set-top boxes 36-44. The control computer 16 provides the request to the appropriate server 46-52 for retrieval of the particular program. As is known, each RAID 18-24 stores a program or plurality of programs in a non-redundant fashion to provide as many selections as possible within the system 10. Accordingly, since each server is associated with a particular RAID 18-24, the appropriate server is addressed to retrieve a particular program.
The control computer 16 also provides the server with the identity of a particular statistical multiplexor 54-60 to utilize. Typically, the server 46-52 will be operably coupled to the statistical multiplexors 54-60 via an alternate space inversion (ASI) connection, which basically includes modulated fixed bit rate binary streams containing MPEG transport streams. Such ASI interfaces may operate at rates up to 240 megabits per second in a point-to-point connection. Such ASI interfaces are expensive, in the range of $4,000.00 per interface.
The statistical multiplexors 54-60 receive a stream of MPEG transport data and provide it to an associated QAM module 26-32. As is known, a statistical multiplexor manipulates input transport streams to generate output transport streams that squeeze more digital data into a single transport stream at a fixed output data rate. Such statistical multiplexors typically support multiple ASI inputs and one ASI output. Currently, a statistical multiplexor is very expensive costing in the range of $40,000.00 per multiplexor.
The QAM modules 26-32 modulate the incoming data into separate analog bands of RF (radio frequency) each occupying a 6 Mhz wide spectrum. Note that the 6 Mhz wide spectrum corresponds to a single NTSC/PAL analog channel. The QAM modules utilize a 256 QAM modulation scheme, which results in a bandwidth of 38.5 megabits per second. As is further known, the rates of an MPEG stream can vary not only based on quantization, frame size and pixels, but that instantaneous data rates within a single stream can vary greatly. Typically, multiple programs are multiplexed together to share the available 3.5 megabits per second bandwidth.
The QAM modulated data is provided to the cable network 14, which demodulates the data and distributes the demodulated data to the requesting set-top box 36-44.
While the video-on-demand system of FIG. 1 provides a stream of video and audio to requesting users, the system is limited by the nature of its design. For example, as the demand for an individual program increases, the video-on-demand system will bottleneck at approximately 12 requests for the individual program. This bottlenecking occurs because of the bandwidth limitation between the servers 46-52 and the statistical multiplexors 54-60 and the bandwidth limitation between the statistical multiplexors 54-60 and the quadrature amplitude modulators 26-32. As previously mentioned, the bandwidth of the path between the statistical multiplexor and QAM is approximately 38.5 megabits per second. In addition, MPEG video data has a rate of approximately 1.5 megabits/sec to 8 megabits/sec averaging approximately 2 megabits per second. As such, each path between a server statistical multiplexor and quadrature amplitude modulation module can support only 12 MPEG video and audio streams.
As is known, MPEG video and audio includes a repetitive pattern of intra frames (I frames), predicted frames (P frames), and bi-directional frames (B frames). Typically, each I frame includes approximately 20 kilobytes of data, each P frame includes approximately 5 kilobytes of data and each B frame includes approximately 2 kilobytes of data. It is further known that the distance between two I frames is known as a group of pictures and that the video material is typically encoded with a constant group of pictures for a given piece of video equipment. In the system of FIG. 1, when multiple streams are multiplexed into a single transport stream, there is a high variability in the data rates of the input streams such that the data rate of the output stream varies greatly. As such, the bandwidth utilization must be derived based on worse case analysis, which occurs when the I frames of each retrieved program are aligned. Other issues with the efficiency of the video-on-demand system of FIG. 1 include program/RAID bottlenecks, server bottlenecks, QAM bottlenecks, statistical multiplexor bottlenecks, scalability, and reliability.
Program/RAID bottlenecks occur when a particular program or group of programs resides on a single RAID disk and multiple users are requesting the program or group of programs. When this occurs, the individual SCSI or IDE interface to the RAID system is limited to 240 megabits per second, thus limiting the number of users which may access the program or group of programs at one time.
Server bottlenecks occur when a particular program or group of programs resides on a single server and multiple user's request the particular program where a server is limited by the ASI output of approximately 240 megabits per second. Thus, to provide a greater bandwidth, a server is required to include multiple ASI interface cards, which dramatically increases the cost of such servers.
QAM bottlenecks occur when a large number of programs are directed to a particular QAM module, which then becomes the limiting factor. A typical solution for this bottlenecking is to use the ASI interface in a multi-drop mode (which is a violation of the ASI specification—DUB-PI EN 50083-9:1998) in order to facilitate sending programs from any of the servers to any QAM module.
Statistical multiplexor bottlenecks occur when each statistical multiplexor typically delivers an MPEG stream of data that includes up to 12 programs to 1 QAM module. As such, for every 12 customers a statistical multiplexor is required. As previously mentioned, such statistical multiplexors are very expensive devices.
The video-on-demand system of FIG. 1 is limited in scalability based on the number of QAM channels and statistical multiplexors. Reliability of the system of FIG. 1 is limited due to the ASI interfaces, which are typically low volume specialty devices that have relatively low mean time between failure ratings. Also, when the ASI interfaces are operated in a multi-drop mode, the reliability of the entire system may be compromised.
Therefore, a need exists for a method and apparatus that provides a scalable, reliable video-on-demand system that overcomes the above-mentioned issues of current video-on-demand systems.